Magnetic ordering temperature

Scientific awareness of a low-temperature transition in magnetite began in 1929 with the observation of a A-type anomaly in the specific heat at about 120 K. The anomaly was typical of an order-disorder transition, but it was well below the magnetic-ordering temperature Tc = 850 In 1931, Okamura observed an abrupt semiconductor-semiconductor transition near 120 K. The transition exhibits no thermal hysteresis, but the transition temperature is sensitive to the oxygen stoichiometry. More recent specific-heat measurements show the presence of two resolvable specific-heat peaks at the transition temperature the lower-temperature peak near 110 K appears to be due to a spin reorientation. 

We now ask what values of V0 and B are likely. Bandwidths in transitional-metal oxides are 1-2 eV the high magnetic-ordering temperature would not lead us to expect a small value of B, if it is given by a term of the type B2jU (Chapter 3, equation (7)). We think that a value of V0 of order 5 eV is very unlikely, in view of the small activation energies involved. It seems much more plausible that the carriers are small (or intermediate) polarons, so that... 

SC — superconducting, AFM — commensurate antifenomagnet structure, SDW — incommensurate antiferro-magnet order (spin density wave), WFM — weak ferromagnetism Tn — magnetic ordering temperature, Tc — superconducting transition temperature and N(Ep) —density of states at the Fermi level. 

Fig. 44. Temperature dependence of the critical field Her (see Kawano-Funikawa 2001) in dependence on the magnetic prehistory of an ErNi2B2C single crystal. ( ) Virgin curve, ( ) H (perpendicular to c) decreasing, (o) H increasing Tc — critical temperature, Tn — magnetic ordering temperature, Twfm — onset temperature...

As can be seen in fig. 47 the temperature dependence of the specific heat C(T) of TmNi2B2C shows pronounced anomalies at the critical temperature Tc as well as the magnetic ordering temperature Tn, which is different from the behaviour of HoNi2B2C where the magnetic contribution to C(T) dominates (fig. 38). 

Fig. 11. Anisotropic thermal expansion of GdCuAl measured by x-ray powder diffraction (Andreev et al. 1999). The lines are extrapolations from the paramagnetic range. 7c and 7r indicate the magnetic ordering temperature and the second magnetic transition observed by Javorsky et al. (1998), respectively.

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